Melanie Gault-Ringold

Anomalous isotopic shifts associated with organic resin residues during cadmium isotopic analysis by double spike MC-ICPMS

Previous studies have reported that matrix effects during analysis by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS), leading to highly anomalous isotopic results, can be derived from ion exchange separation procedures, as resin-derived organics are stripped from the column together with the element of interest. In low concentration samples, where matrix to analyte ratios are high, these artifacts cannot be corrected for using sample-standard bracketing or external normalization techniques to monitor instrumental mass fractionation. It has been suggested that such artifacts can be overcome by implementing a double spike for the correction of instrumental mass fractionation. We present new Cd isotopic results showing that although a double spike reliably corrects for instrumental mass fractionation during sample analysis, the effects of these resin-derived organics introduce very anomalous shifts in bracketing standards that have not been chemically processed using ion exchange resins. These inaccurate results appear to be the consequence of one or more polyatomic interference/s or mass independent isotopic fractionation as opposed to the previously proposed matrix effects. These analytical artifacts can be completely alleviated by the oxidation of samples with solutions of either H2O2/HNO3 or HClO4/HNO3 prior to analysis. Although the present results have direct relevance for Cd analysis, it is probable that other isotopic systems are affected by similar resin-derived organics. The increased sensitivity of the next generation of MC-ICPMS instruments means that progressively smaller sample sizes are accessible for analysis, leading to a larger proportion of resin-derived matrix in many samples, such that oxidation of samples will prove to be essential to circumvent the issue of inaccurate isotopic measurement.

Insights Into the Biogeochemical Cycling of Iron, Nitrate, and Phosphate Across a 5,300 km South Pacific Zonal Section (153°E–150°W)

Iron, phosphate and nitrate are essential nutrients for phytoplankton growth and hence their supply into the surface ocean controls oceanic primary production. Here, we present a GEOTRACES zonal section (GP13; 30-33oS, 153oE-150oW) extending eastwards from Australia to the oligotrophic South Pacific Ocean gyre outlining the concentrations of these key nutrients. Surface dissolved iron concentrations are elevated at >0.4 nmol L-1 near continental Australia (west of 165°E) and decreased eastward to ≤0.2 nmol L-1 (170oW-150oW). The supply of dissolved iron into the upper ocean (<100m) from the atmosphere and vertical diffusivity averaged 11 ±10 nmol m-2 d-1. In the remote South Pacific Ocean (170oW-150oW) atmospherically sourced iron is a significant contributor to the surface dissolved iron pool with average supply contribution of 23 ± 17% (range 3% to 55%). Surface-water nitrate concentrations averaged 5 ±4 nmol L-1 between 170oW and 150oW whilst surface-water phosphate concentrations averaged 58 ±30 nmol L-1. The supply of nitrogen into the upper ocean is primarily from deeper waters (24-1647 μmol m-2 d-1) with atmospheric deposition and nitrogen fixation contributing <1% to the overall flux, in remote South Pacific waters. The deep water N:P ratio averaged 16 ±3 but declined to <1 above the deep chlorophyll maximum (DCM) indicating a high N:P assimilation ratio by phytoplankton leading to almost quantitative removal of nitrate. The supply stoichiometry for iron and nitrogen relative to phosphate at and above the DCM declines eastward leading to two biogeographical provinces: one with diazotroph production and the other without diazotroph production.